text: The count rates observed from a given astronomical radiation source have no simple relationship to the flux from that source, such as might be incident at the top of the Earth's atmosphere. This lack of a simple relationship is due in no small part to the complex properties of radiation detectors.
  These detector properties can be divided into
  *those that merely attenuate the beam, including
  *#residual atmosphere between source and detector,
  *#absorption in the detector window when present,
  *#quantum efficiency of the detecting medium,
  *those that redistribute the beam in detected energy, such as
  *#fluorescent photon escape phenomena,
  *#inherent energy resolution of the detector.